A U.S. federal judge says he may order Hillary Clinton to testify in a lawsuit concerning her use of a private email server for official business while she was secretary of state.

A conservative group called Judicial Watch is suing the State Department for access to some of the e-mails Clinton sent while secretary of state.

It contends the State Department did not release all the details it was seeking under the Freedom of Information Act on former department employees who worked with Clinton to set up her email account.

District Judge Emmet Sullivan said Wednesday it "may be necessary" to summon Clinton to testify about the circumstances that led the State Department approval for her using a private email server for official business.

Clinton has consistently denied any wrongdoing and says she used one email account for personal and official use for convenience.

Clinton critics, including Republican presidential candidate Donald Trump, have accused her of putting national security at risk.